The present invention relates to portable drill units that are provided with a magnetic base, suction cup or other device to attach the drill unit to a work surface. More particularly, the present invention relates to a torque restraining device for resisting movement of the drill unit during operation. For clarity, the invention will be described with respect to a drill unit having an electromagnetic base. However, it should be understood that the electromagnet could be replaced by suction cups or equivalent means to make the portable drill unit self attaching.
Basically, a magnetic base drill is a portable drill press. It includes an electromagnet, a support affixed to the electromagnet and an electric drill motor reciprocally mounted to the support member so that it can be raised and lowered with respect to the work surface. The electromagnet is energized to create a magnetic flux between the electromagnet and the work surface to magnetically adhere the magnetic base drill to the work surface. In this way, holes may be drilled in a work surface at remote locations where a standard drill press could not be taken. Common uses for magnetic base drills are in the construction and repair of bridges, high-rise buildings, etc.
The torque of a magnetic base drill causes a twisting or torsional force which must be resisted by the magnetic base of the drill. The motor of the magnetic base drill produces high torques when loaded down, tremendous torques when bogged down and even greater torques when the motor is stalled. The highest torque, the torque obtained when the motor is stalled, is called the stall torque and occurs when the rotation of the motor is stopped or stalled in the work. This stall torque is significantly higher than normal operating torques of the motor. Unless the magnet of the magnetic base drill resists all of these torques, the magnetic base drill may slip or even break away from the work surface and spin out of control.
The electromagnet of a magnetic base drill creates a strong normal force to attract the drill to the work surface, but consderably less force to resis rotation. With standard magnetic base drills, not including a torque restraining device, the electromagnet cannot adequately resist this twisting rotation resulting in inadvertent movement.
An early attempt at solving the problem of inadvertent rotation of magnetic base drills is disclosed in U.S. Pat. No. 2,622,457 to Buck. Buck attempted to solve the problem by using two magnets spaced from one another. The difficulty with this attempt is the fact that magnets have very little resistance to torsional movement. Even though one of the magnets is spaced a distance from the other, the resistance to torsional movement is not greatly enhanced.
United States Patent No. 4,261,673, issued to Everett D. Hougen discloses a solution to the problem of inadvertent rotational movement. The solution involves the driving of a torque restraining device into the work surface to prevent rotational movement. It was discovered by Mr. Hougen that the amount of penetration needed to resist rotation is relatively small, even though the stall torque of the motor was very great. Mr. Hougen discovered a synergistic effect between a torque restraining pin that penetrates the work surface and the magnetic attraction of an electromagnet. Neither the magnet nor the torque pin when used separately provided enough resisting torque. When used together, the resistance torque was greater than the sum of the torque resistance provided by the pin and magnet and was found to be enough to prevent rotation and resist the stall torque of the motor. With a slight penetration of the pin and the force of the electromagnet, the resistance to rotation is dramatically increased over the available resistance from the electromagnet alone. As disclosed in the '673 patent, there are several different ways to drive the torque restraining device into the surface.
In addition to the torque restraining pin, the '673 patent discloses the use of a roller to permit the magnetic base drill to be easily aligned over the place where a hole is to be drilled. (This roller is also disclosed in U.S. Pat. No. 3,969,036.) Many times, holes must be drilled at exact predetermined locations and alignment is crucial. By using the roller, the drill can be easily slid across the work surface to each location and aligned. Further, the ability to slide the magnetic base drill across the work surface clears away metal chips which have been left on the surface during drilling. The front of the magnetic base drill with the rear raised by the roller acts as a scraper to clear the work surface of metal chips to provide a better surface for the magnet to adhere.
A disadvantage of the torque restraining device of the '673 patent is the exposed torque restraining pin. If the magnetic base drill is handled roughtly, there is the possibility that the torque restraining pin may be dulled. The penetration of the torque restraining pin may be only about 0.015 inches, and therefore a dull or chipped pin may be insufficient to restrain the torque generated. Commonly, the operator's manuals for drill presses of this type advise the operator to maintain the sharpness of the pin.
A further disadvantage of the '673 patent is the use of a separate torque restraining device and plunger. This requires a larger mounting area which increases the size, weight and manufacturing cost of the magnetic base drill. Still further, positioning the torque restraining device and/or plunger of the magnetic base drill center line can be disadvantageous in some applications. By combining the torque pin and glide or protective member into one unit both can be located where they are most effective, for example, along the center line of the magnet when used on tubing.